An Ultrastructural Study on the Mode of Attachment in Dinoflagellates Invading Gills of Cyprinodontidae

Résumé du livre

The ultrastructure of two ectoparasitic dinoflagellates of two genera was studied with respect to their mode of attachment. Trophonts of Oodinium cyprinodontum Lawler, 1967 and Amyloodinium sp. Differed in the way they attached to the gill epithelium of cyprinodontid fishes from estuarine waters and in their nutrition. The cytoplasm of O. cyprinodontum is characterized by numerous typical dinoflagellate chloroplasts and shows no traces of ingested host material. The trophont adheres to the gill filament by a flattened part of the cell which ramifies into major and minor branches that bear minute finger-like rhizoids, the tips of which are attached to the surface of the host cells. At the point of contact, the host cell membrane is altered, but no penetration of rhizoids into the cell could be observed. The photosynthesizing O. cyprinodontum appears to be a symphoriont rather than a true ectoparasite. Its generic classification has to be changed. Amyloodinium sp. completely lacks chloroplasts; the subnuclear part of the cytoplasm of the trophont contains numerous food vacuoles, filled with ingested host cytoplasm. At the base of the trophont there is a gap in the thecal armour, encircled by an osmiophilic ring which is connected with an attachment plate bearing numerous filiform rhizoids. These rhizoids are embedded in the gill epithelial cells and inflict heavy damage on invaded cells. A long conspicuous stomopode is associated with the attachment plate; it has an axial fibrillar tube that passes through the osmiophilic ring deep into the trophont. This tube contains various kinds of vesicles and clove-like bodies of supposedly enzymatic function. The flagellar canal opens into a rudimentary sulcal fold at the base of the trophont. It is connected with the pusule and may also be the site of food ingestion. The mode of ingestion of host cytoplasm is discussed. The pathogenicity of Amyloodinium may be explained by its destructive effect on the host tissue.